1. Field of the Invention
The present invention relates to a method for pulling crystal which can set a space between the surface of a melt and a radiation screen when pulling crystal, such as silicon single-crystal, by a Czochralski method.
2. Description of the Related Art
Generally, crystal such as silicon single-crystal, is pulled from melt in a crucible made of high-purity quartz glass. It is called Czochralski method. At this time, the quartz glass is melted into the silicon melt, and oxygen atoms are diffused into the melt and brought into the single-crystal. Furthermore, silicon oxide (SiO) is evaporated from the surface of the melt.
On the other hand, recently, since silicon wafers used for manufacturing integrated LSI have been underway, further more precise control on oxygen concentrations of the silicon wafers is demanded. The variation of the space between the surface of the melt and the radiation screen is considered a factor to cause less precise control on oxygen concentrations. Therefore, a method for improving the space control more precise is required.
"Apparatus and Method for Pulling crystal", serving as an improved method for solving this problem, was disclosed in Unexamined Japanese Patent Publication (kokai) No. Hei 7-330484. In this publication, a quartz glass-made measuring device for measuring the position of the surface of a melt is contacted with or dipped into the melt, thereby measuring the height of the surface of the melt by an image process.
However, in practice, to measure the scale of the quartz glass-made measuring device or the dipped part thereof is difficult. Moreover, when the measuring part is at a temperature above 1400.degree. C., contrast is weakened owing to intense radiation. Hence, a high-cost and high-quality image processing device is necessary.
Furthermore, the quartz glass-made measuring device which is easily softened and deformed at a temperature above 1400 .degree. C., is unable to be used to serve as a measuring device.
In addition, since the measuring device is continuously dipped in the melt under this condition, some crystals grow from the front end of measuring device, so, those disturb correctly growth of single crystal.